Token-ring networks conform to the IEEE 802.5 standard, and include a number of stations (or nodes) connected in a closed-loop ring network, within which a token is circulated from station to station. For a station to communicate over the ring network, it must have priority to the token, and will accordingly take the token from the ring when it is available, and transmit a signal indicating that the token has been taken. At this point, no other station may communicate over the network, and the token-holding station (i.e. the source station) transmits a data frame to a destination station. The data frame will be propagated in one direction, and from station to station, around the ring network until received by the destination station, which copies the data frame into internal storage, and forwards the message on. Once the source station again receives the data frame that it generated, it releases the token for use by other stations.
Each station within a token ring network thus acts as a repeater for token and message data frames. When a new station is added to the ring network, it undergoes an initialization sequence to become part of the ring network. It will be appreciated that it is crucial to proper functioning of the ring network that the closed-loop ring network be maintained at all times, and that all stations and cabling be functioning properly. The failure of a single station or its connections can cause the entire ring network to be rendered inoperative.
The integrity of a ring network is particularly vulnerable during the addition and removal of stations and other devices. For example, the insertion into a token ring network of a station that violates the IEEE 802.5 protocol, or that has a broken receive/transmit cable, can render the whole token ring network inoperative.
A number of methods of locating and isolating faults within token ring networks have been proposed. These methods range from manually locating and replacing faulty stations or cables, to more sophisticated methods involving a network probe and isolation circuitry. Examples of such sophisticated techniques are described in U.S. Pat. Nos. 5,283,783 and 5,361,250, both entitled "Apparatus and Method of Token Ring Beacon Removal for a Communication Network", and U.S. Pat. No. 5,508,998, entitled "Remote Token Ring Beacon Station Detection and Removal". While the methods and apparatus described in these references are effective to located and isolate faults within a token ring network, they require that the network become inoperative while the location and isolation process is occurring. The time required to perform these methods is also often unacceptably long.